Welcome to my Doubledoku Solver

Use the main board to enter clues. Make sure you select "Enter clues" and change it to "Enter Solutions" when you come to manually solving it.

All feedback, comments, arguments, bug reports and strategy ideas are welcome. There is a FEEDBACK form with a column displaying comments and questions. Many thanks to all the people who have done so and helped improve this solver.

New: Candidate and cell colouring

Strategy Overview

> Tick and untick the check boxes to enable/disabled specific strategies. The first five 'basic' tests are the simplest and my advice is to allow the solver to use them as skipping may cause unexpected logic in later strategies.

> The order of these advanced strategies - and my inclusion of them in categories 'tough', 'diabolical' and 'extreme' are my personal choice after close study and are roughly in order of complexity. While the logic is different for each, you should be aware that there is considerable overlap in their power to solve in certain situations. For example, X-Cycles are a subset of Alternating Inference Chains. If you turn off X-Cycles the same elimination might turn up under AICs. Since March 2010 I have reworked most chaining strategies to find the best and shortest chains - not necessarily the first one it happens to find.

> All strategies in the list have links to documentation, but it's worth describing what the first tests do:

• Show Possibles (Naked Singles): For each unknown square we eliminate all possibles where those numbers are known in each row, column and box. This may reveal a single candidate, in which case we have a solution for that cell.
• Hidden Singles: If a candidate occurs once only in a row or column or box we can make it the solution to the cell.
• Test 2: In this test we check for 'naked' Pairs and Triples. For example, if we have two pairs, eg 3-4 and 3-4 in the same row, column or box, then both 3 and 4 must occupy those squares (in what ever order). 3 and 4 can then be eliminated from the rest of the row, column or box.
• Test 3: Naked and Hidden Quads - much rarer but simply extends Pairs and Triples.
• Test 4: See Pointing Pairs and Triples for a full explanation. This test helps us eliminate numbers in rows and columns outside the box.
• Test 5: Box/Line Reduction. We check the box against the rows and columns that intersect it for each number.
• Test 6: This test is for Hidden Pairs, Hidden Triples